Fixed #18269 -- Applied unicode_literals for Python 3 compatibility.

Thanks Vinay Sajip for the support of his django3 branch and
Jannis Leidel for the review.

tests/regressiontests/admin_widgets/models.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 from django.db import models
 from django.contrib.auth.models import User
 
@@ -60,7 +62,7 @@ class Car(models.Model):
     model = models.CharField(max_length=30)
 
     def __unicode__(self):
-        return u"%s %s" % (self.make, self.model)
+        return "%s %s" % (self.make, self.model)
 
 class CarTire(models.Model):
     """